The invention resides in a drive unit for a wheel or wheels of a vehicle which is operated at least partially by an electric motor.
DE 20 2009 014 347 U1 discloses, for example, a bicycle which, in addition to the pedal crank drive, is provided with an electric motor which drives the rear wheel and which supports the pedal power of the driver. The electric motor is disposed in the rear wheel hub.
Also, DE 602 09 510 T2 which is mainly concerned with the control of an electric motor or a bicycle favors a wheel hub drive.
Although wheel hub drives are a common solution for the implementations of an auxiliary drive or possibly also a main drive, they have a substantial weight and they require either an exchange or a re-spoking of the wheel which is to be provided with a drive unit. They are therefore not particularly suitable for retro-fitting. In particular, a combination of a hub motor with a chain gear shift mechanism as it is provided generally with sporty bikes and which is favored by sport-oriented drives is difficult and possible only within limits.
Especially sport-oriented drivers often own valuable bikes which do not lend themselves to an installation of wheel hub drives of any type. The driver may also find it objectionable that the electric auxiliary drive prevents the wheel from rotating freely when the motor is not energized that is when the motor runs neither as motor nor as generator. This is particularly true for motors with permanent magnet excitation which are generally used because of their high efficiency.
Based hereon, it is the object of the present invention to provide an auxiliary electric drive for a vehicle wheel, in particular a bicycle especially for retrofitting.